Power collector for inductive power transfer

ABSTRACT

This inductive power transfer system provides a capacitor (15) having a pickup coil (14) in series between the windings of the pickup coil and the electrical load (16, 17, 18). A power-factor compensating capacitor (19) may be used at the feed to a loop of one or a few turns of primary inductor cable (11,12), so that a reasonable supply-mains current can maintain the primary inductor current. The length of the cable, and hence the impedance, of primary cable may be varied in order to match the primary inductor to the voltage of the supply mains.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a system for inductive transfer of poweracross a gap, useful in the provision of electric power to one or moremobile consumers of electric power. It has particular application to anelectrically powered railway or to road vehicles.

BACKGROUND

The concept of inductive coupling as a roadway power transmission systemhas been known for approximately 100 years, and there have been manyattempts to develop the use of inductive coupling for the transportationof road vehicles. However, they have not proved to be sufficientlyefficient and hence not commercially successful.

OBJECT

It is an object of this invention to provide an improved inductionsystem capable of powering vehicles, or one which will at least providethe public with a useful choice.

STATEMENT OF THE INVENTION

In one aspect the invention comprises a power collector for collectingpower from an inductive power transfer system across a space, using asits primary emitters of power as varying magnetic flux one or moreprimary conductors capable of carrying an alternating electric currenthaving an operating frequency, the power collector comprising at leastone capacitance and at least one coil of one or more windings forintercepting the magnetic field from the one or more primary conductors,and at least one electrical load, characterised in that the capacitanceis in series with the coil and the load, and the reactance of the seriescapacitance is substantially equal to the inductive reactance of thecoil at the operating frequency.

In a subsidiary aspect the windings of the coil of the power collectorare made of solid conductive metallic material.

In a further subsidiary aspect the electrical load comprises a devicecapable of immediately consuming the alternating current developedwithin the windings of the coil.

In an alternative subsidiary aspect the electrical load comprises adevice capable of using power from a storage battery connected to thepower collector.

In a yet further subsidiary aspect the load comprises a device capableof using the alternating current after a first conversion into directcurrent and a second conversion into a voltage or current of a regulatedamplitude.

In a more direct subsidiary aspect the load comprises a DC motor capableof providing a motive power .

In a second main aspect the invention comprises a primary conductor (foruse with a power collector), comprising an elongated loop formed fromone or more turns of an insulated conductive cable of a type having, forthe conductive portion, a low ratio of external surface are to volume.

In a related aspect the conductive cable may optionally comprise asuperconductive material of a type capable of exhibiting a very lowohmic resistance when cooled to below its superconducting temperature.

In another aspect the invention provides an inductive power transfersystem in combination with a power collector as previously described,said system comprising a source of alternating electric power having anoperating frequency; a resonant primary; a vehicle having at least onesaid power collector mounted thereon; wherein said power collectorcomprises at least one capacitance and at least one coil of one or morewindings for intercepting the magnetic field from the one or moreprimary conductors, and at least one electrical load, characterized inthat the capacitance is in series with the coil and the load, and thereactance of the series capacitance is substantially equal to theinductive reactance of the coil at the operating frequency.

In another aspect at least one metallic rail may be used as a primaryconductor, upon which a substantially non-conductive vehicle may besupported by at least one non-conductive support incapable of conductingan electric current from the rail.

In a further aspect the operating frequency of the source of alternatingpower used in this system lies in the range of from 30 Hz to 1 MHz.

More preferably the operating frequency of the source of alternatingpower is that of a local mains supply frequency or an integer multipleof the supply frequency.

In a yet further related aspect there is more than one, electricallyseparated, elongated and looped primary conductor; each loop beingcontiguous at at least one end with an adjacent loop.

Alternatively one or more physically separated primary loops are locatedat designated positions.

In a further aspect, the invention may comprise a self-powered vehicle,adapted for use with an inductive power transfer system, comprising atleast three road wheels and means for control and guidance, at least onepower collector, apparatus for conversion of electrical energy into astorable form, apparatus for storage of electrical energy, and apparatusfor recovery of stored electrical energy and its controlled delivery toat least one electromotive transducer.

Other subsidiary aspects will become apparent from the claims and thetext of this specification.

DRAWINGS

These and other aspects of the invention, which should be considered inall its novel aspects, will become apparent from the followingdescription, which is given by way of example only, with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic diagram of an inductively coupled railway withseries capacitor.

FIGS. 2a and 2b illustrate a cross-section through a pair of rails, andthrough an electrically powered trolley positioned on said rails.

FIG. 3 is a top plan view of a coil positioned above said pair of rails.

FIG. 4 illustrates a simple resonant circuit to illustrate the powerpick-up in the coil of the vehicle.

FIG. 5 illustrates a series resonant circuit for use in the coil of thisinvention.

FIG. 6a illustrates a vehicle with on-board storage at a designatedcharging place.

FIG. 6b illustrates a preferred primary conductor loop.

FIG. 6c illustrates a superconducting primary conductor loop.

FIG. 7a illustrates a vehicle at a designated charging place,illustrating the relationship of the primary windings and the powercollector.

FIG. 7b illustrates a section through a power collector.

EXAMPLE 1

Alternating current is supplied to a pair of parallel conductors (inthis case a pair of electrically conductive rails 11). The frequency mayvary from about 50 Hertz (50 Hz) to several MegaHertz (MHz) dependingupon the size and nature of the track and trolley, and the relativeeconomics of high frequency generation over frequencies related to mainsfrequencies.

Preferably the power is supplied at a frequency in the range of 2KiloHertz (KHz) to 20 KHz. The prototype described in the preferredembodiment, and in the calculations, made use of a power supply of 10KHz. The frequency and the current developed by the power supply may beselected, or varied, depending upon the size and nature of the rails,the size and nature of the pick-up coil, and the power requirements ofthe electric motor or motors on each trolley.

The induction system of this invention is particularly suited to anelectrical railway in which a pair of electrically conductive rails areused to support and guide one or more vehicles on the railway, and atthe same time serve as a pair of parallel conductors for the alternatingcurrent supplied to the railway. These rails emit a varying magneticflux which is the medium by which power is transferred to the powercollector, more particularly to a secondary winding within it. Theserails 11 are illustrated in FIG. 1. Referring to FIG. 1, the rails 11are short-circuited at 12, and supplied by a power supply 13 providingalternating current. The pick-up coil 14 of a trolley is connected via aseries capacitor 15 to a rectifier 16 and a DC--DC converter 17 tosupply a brushless DC motor 18.

As shown in FIG. 2, it is preferred that the rails 1 1 are formed from agood electrical conductor such as aluminum, and this may be in the formof a solid or hollow section bar. In a prototype of this invention, thepreferred rails comprise hollow aluminum rails, of outside dimensions of30 mm×30 mm, having a central cavity, leaving a wall thickness ofapproximately 3 mm on either side This construction is suitable forlightweight trolleys or cars supported on the rail system In thisprototype, the spacing of the rails is 600 mm. Such a rail spacing issuitable for carts of approximately 800 mm long×600 mm wide, as thisdimension is relevant to the size of the coil which can be placedbeneath the cart in close proximity to the rails.

As will become apparent from the calculations in the example, the sizeof the coil and the frequency of the alternating current supplied to therails enables the prototype unit to provide approximately 400 watts toan electric motor situated on the trolley.

The rails are preferably supported on non-electrically conductivematerial, in order to avoid shorting out the rails, or the dissipationof the inductive power into metallic components close to the rail. Forexample, the rails may be directly attached to an inert substrate, ormay be supported or separated by insulated sleepers such as timber, orplastic sleepers. In designing an inductively powered railway, it ispreferred that the path of the railway is kept free of other metals, andin particular it is preferred that a space on either side of the rails,and above and below the rails equivalent to the rail spacing issubstantially free of metal (other than the pick-up coil on thetrolleys) in order to minimize inductive losses. As there is no directelectrical contact between the wheels and the rails, it is possible tocoat the surface of the rails with an insulating or protective coating,such a coating may reduce transport noise in the system.

Alternatively, the wheels may be coated with, or made of, a plasticssubstance or another non-conductive material

The trolley 20 shown in FIG. 2 preferably has a minimum of metal inclose proximity to the powered rails, so that most of the power suppliedby the rails is picked up by the coil 14 in close proximity to therails, rather than by other metal components on the trolley. It is forthis reason that the wheels 21 are preferably formed of a non-conductivematerial, such as a plastics material, although the axle 22 connectingthe wheels may be a metal axle. As shown in FIG. 2, the pair of wheelsof the trolley (of which there would normally be four wheels) areconnected by a solid axle, typically a metal axle 22 supported on a pairof bearings 24. The wheels themselves will normally be formed of ahardwearing plastics material, although in the prototype trolley thewheels were machined from a high density particle board.

The deck 26 of the trolley is preferably formed of wood or plastics, oranother nonconductive material, and an electric motor 27 is positionedunderneath the tray, preferably as far away from the rails as possible,and towards the center of the tray, approximately equidistant fromeither rail. Any convenient form of electric motor may be used, althoughin the prototype system, we prefer to use a brushless DC motor, withvariable speed control. The electric motor is preferably connected tothe axle by means of a pair of pulleys 28, 29 connected by a belt (notshown).

The coil is preferably suspended beneath the trolley as close aspossible to the rails although a clearance of 50-100 mm is acceptable.In the prototype a distance of 25 mm has been allowed between the outeredges of the coil and the adjacent rail. As shown in FIG. 2 the wheelsare preferably flanged wheels, and as is typical with railway systemsthe flanges are provided on the inside of the rail. This has workedsatisfactorily in the prototype trolley, although if a closer couplingbetween the coil and the rail is desired, then an alternative wheelconfiguration may be used, for example, the wheels may be reversed sothat the flanges sit on the outside of the rails, allowing the coils tobe placed very close to the rails without the flanges interveningbetween the coils and rails. Other wheel and rail designs are possible.

In the prototype, we have used a power supply capable of supplying analternating current of 10 KHz, at 100 amperes. The supply circuit 13(see FIG. 1) is preferably provided by means of a pulse width modulationintegrated circuit such as a Texas Instruments TL494 chip.

The pick-up coil is approximately 600 mm wide and 800 mm long(corresponding as closely as possible to the overall dimensions of thetrolley). In the prototype we have used a 50-turn coil of 1 mm copperwire resulting in an induced current of approximately 2 amps at 200volts in the pick-up coil, providing approximately 400 watts to themotor.

In cross-section the prototype coil is approximately 8 mm wide and 10 mmdeep, made up of 50×1 mm diameter insulated copper wires. This is shownas 14' in the expanded view in FIG. 2b.

The pick-up coil preferably forms part of a series resonant circuit,connected to a DC--DC converter to control the output from the coil sothat it is suitable for driving a brushless DC motor. The DC--DCconverter may be of the buck converter type

In most cases the frequency of the power supply to the rails will befixed, although if desired, a variable frequency source could be used,particularly if additional demands are made on the system, eg if moreheavily-laden trolleys are used at some times rather than others.

The system allows for one or more trolleys to be powered by the rails,and if desired each trolley can be independently controlled, so that itsspeed and location can be controlled by controlling the brushless DCmotor on each trolley. Intelligent controllers could be provided on eachtrolley to monitor the position of each trolley, and signals could besupplied via the rails to each trolley, and from each trolley to acentral controller, if desired.

The series resonant circuit of FIG. 5 is also shown schematically inFIG. 1, and is connected to a rectifier, and to a DC--DC converter toprovide speed control to a brushless DC motor This brushless DC motor ispreferably connected to an axle directly coupled to a pair of wheels, sothat if a trolley has four wheels, two will be driven, and the otherpair will be allowed to free-wheel.

In its preferred form the invention allows the pick-up coil to be of asize close to the overall size of the trolley, ie the trolley willpreferably be rectangular in plan view, as will the coil. By using therails as conductors, the coil can be close to the full width of thetrolley. This construction has the advantage that no further wiring needbe applied to the rail tracks.

Calculations for Example 1

In the following example reference is made to the parallel rail shown inFIG. 1, and to the schematic diagrams shown in FIG. 3, 4 and 5. In thefollowing calculations reference will be made to a pair of rails(parallel conductors) at a spacing of 1 meter between them, for ease ofdemonstration.

Taking 2 parallel conductors at a spacing of 1 meter between them, thenthey have an inductance L=1.2 microhenries/meter.

If a rectangular coil of wire is placed close to the parallel wire, asin FIG. 3, a movable transformer is created where the stationeryparallel wires are the primary winding and the secondary winding is thecoil of wire of N turns, with length l meters.

Therefore the area of mutual inductance between the primary andsecondary remains constant as the secondary is moved along the length ofthe primary winding.

For simplicity let N=1 and l=1 meter, then we have a secondary windingof 1.2 microhenries.

Now if the frequency of the supply is, say 10 KHz, and it is applied toone end of He primary winding and the other end shorted, then thefollowing equations apply:

    Z.sub.L jπ×1.2×10.sup.-6 ×l.sub.p Z.sub.L =impedance in ohms

let

    l.sub.p =10 meters l.sub.p =length in meters

    Z.sub.L =j2π×10.sup.4 ×1.2×10.sup.-6 ×10 ω=2πf

    Z.sub.L =j0.754 ohms f=10 KHz

Now if a current of 100 amperes is passed through the primary section,then a voltage of 75.4 volts will occur across the input. Thiscorresponds to 7.54 volts/meter and therefore by mutual inductance 7.54volts will appear across the 1 turn secondary winding.

Now if the secondary winding is loaded as shown in FIG. 4, the impedancewill limit the output voltage and therefore output power as follows:

    V.sub.S =I.sub.S (R+jZ.sub.L)

to obtain maximum power transfer R=Z_(L) (see FIG. 4). ##EQU1##

To increase the power transfer a series resonant capacitor can be placedin the circuit as shown in FIG. 5.

If jZ_(L) =-jZ_(c) then the output impedance will be zero but currentlimiting occurs due to the impedance of the remaining uncovered sectionof the primary.

Therefore by transformer action maximum current in the secondary will be100 amperes (if 100% coupling occurs), Therefore maximum output poweroccurs ##EQU2## which is double the previous maximum output powerpossible without resonance.

In the example given then the maximum ##EQU3##

The increase of resistance due to "skin effect" should also be takeninto account. An aluminium conductor of 3 mm diameter will have a 10%increase in resistance at 10 KHz. Therefore care must be taken incalculating the actual resistance of conductors used. In one experiment,a 12 mm by 12 mm aluminium busbar was used as a combined rail andprimary conductor for a powered rail system.

This gave a measured AC resistance of 14 milliohms for a 10 metersection and losses of 200 watts at a current of 100 amperes.

EXAMPLE 2 FIGS. 6a, 6b, 6c, 7a, 7b

This embodiment describes an inductive power transfer system 60 adaptedfor use with a vehicle, specifically a small (20-seats) bus 70 used fora short route up and down a shopping street. The vehicle or bus isplanned to collect energy at charging stations 71 (usually identical tobus stops) along its route, where sources 62, 602, 76 of varyingmagnetic flux are provided, and store it for use as motive power betweenstops. This vehicle and its inductive supply systems are optimized forsimplicity rather than absolute efficiency.

This bus has a conventional arrangement of steerable rubber-tired wheelsat the front and a pair of driven rubber-tired wheels 74 at the rear. Itmay be advantageous to have all four wheels steerable, to better serveparticular applications.

The bus covers a road area of 7 meters in length, and 24 meters inwidth. A power collector according to embodiments described earlier maybe installed as a "skirt" around the perimeter of the bus (optionallycurving up over wheel arches) and is provided as described in earlierembodiments with a series resonant capacitor of a size such that itsreactance is substantially the same as the inductive reactance of thewindings of the power collector. Alternatively the power collector maybe inside the line of the wheels as shown by 77. The calculationsincluded herein relate to this example and clearly may be varied fordifferent conditions--such as using a 117 V AC, 60 Hz supply.

Calculations for Example 2

Power Collector:

Coil dimensions: 5 m length×2.4 m, rectangular.

Wire: 100 turns of 2.0 mm copper. ##EQU4## (This voltage depends on thenumber of turns used and may be altered according to requirements.)

As the coil resistance is 8 ohms, then maximum power transfer occurswhen ##EQU5##

Normally, to keep efficiency higher, the current may be limited to 10 A,whereupon the output power from the collector

=400-(10×8)

=320 V AC

P_(out) =3.2 KW.

Primary Loop or Coil:

1000 A are provided in effect by a 9-turn primary loop, if the conductorcurrent is 110 A. ##EQU6##

Therefore a 30 meter track could be run directly off a 240 V AC mainssupply.

(30×8=240)

Power factor correction using a capacitor across the input to the loopreduces the 110 A of circulating current to about 10 A of supplycurrent. ##EQU7##

Track losses (I² R), assuming 95 mm² aluminium rod as the primaryconductor, are: ##EQU8##

Up to three 10 meter long buses could be charged at one time. For onlyone bus, the total losses are:

P_(L) =1.92 Kw+4 Kw

Power out P_(O) =3.2 Kw

System efficiency η%=3.2/4.92=65%

The storage device is a relatively small number of deep-dischargelead-acid storage batteries 63 (although any other type of storagebattery may be used). The number of batteries preferred for any onevehicle is dependent on factors such as battery cost, durability (interms of charge/discharge cycles) as against the feasible operatingvoltage, charging rate (and hence time), the distance between stops,battery weight and cost, and safety margins (such as a particularrecharging stop being unavailable).

A suitable battery charger/battery monitor device is provided to rectifyand optionally to change the voltage of the current from the powercollector and charge the batteries whenever power is available, and toindicate the rate of charge and the state of charge. Such devices arewell known in the art.

The preferred electromotive transducer is a type of brushless DC motor,preferably of an ironless construction such as a "CADAC" motor,preferably having an output of about 30 KW, and preferably a singlemotor is coupled to the drive shaft of a conventional differentialgearbox such as a two-speed "Eaton" differential. Each motor has its owncontroller to energize windings in the correct order, and usually thisis driven by a variable DC supply derived from the battery voltage inorder to provide a variable motor torque or speed. Other types of motormay be preferred.

This vehicle design may have the usual features of electric motor drivesuch as regenerative braking, although the initial prototypes havedeleted these features as the vehicle runs on a flat area in a notoverly congested route.

Alternatively a motor (and its own controller) could separately driveeach wheel, coupled through (for example) a planetary gearbox providinga geared-down ratio to the wheel hub. A further alternative is the useof multiple pairs of drive wheels, each pair having its own motor anddifferential. The low speed of the differential may be required forextra driving torque.

An option intended to reduce track losses is the use of superconductingmaterials to carry the large currents. Such an arrangement isillustrated in FIG. 6c, where 69 is an array of superconducting primaryinductors 62c together with pipes for coolant 68 buried withininsulation material 603 in a lined trench 602 under a roadway 601. Newlydeveloped superconducting materials based on copper oxide and yttrium(for example) are capable of exhibiting superconductivity atcommercially realisable temperatures--above the boiling point of liquidnitrogen.

Each designated charging position would comprise for example aconventional curbside bus stop 71. A shallow channel 67b cut in arectangular form into the roadway at a position which is intended to liesubstantially beneath the power collector 77 of the stopped bus 70 isfilled with loops of the primary conductor; preferably in the form ofsingle-conductor, underground cable of the type comprising a single 12mm diameter aluminium rod sheathed with double insulation and approvedin New Zealand for use at up to 400 V AC. This type of insulatedconductive cable has a relatively low ratio of external surface area tovolume for the conductive portion, in contrast to multiconductor cables.Preferably the length and number of turns are planned as outlined in thecalculations above to provide a directly mains-connectable primaryinductor, thereby discarding a transformer. 65 is the mains input supplyin FIG. 6a, feeding through a controller 61 (including a power factorcorrection capacitor (as 19) and a contactor) and through wires 66 tothe primary loops 62 (or 62b, or 76). In effect, the power factorcorrection capacitor renders the primary loop a parallel resonantcircuit carrying a circulating current about ten times larger (in oneinstallation) than the injected current. Alternative installations mayuse frequency-doubling techniques or otherwise raise the frequency ofthe primary alternating current, as the coupling is improved by thesquare of the operating frequency, but it is believed that the economiccost and costs in terms of reliability of such improvements are greaterthan using more turns of cable (now at NZ $2.50 per meter).

In order to minimize wasted power, the primary conductor is preferablyenergised only when the correct type of vehicle is over the primaryconductors. Various indicators may be provided on the bus to indicate tothe driver the best placement of the vehicle over the buried primaryconductor--indicators such as a simple voltage or power meter, orexternal physical locators.

The ironless inductive power transfer system of the preferred embodimentworks effectively with coil separation of the order of 100 mm (between76 and 77 in FIG. 7a) and, given a relatively stiff suspension for thebus and no humps in the roadway along the routes travelled, there maynot be any need to provide means to physically lower the power collectorsupport 78 into closer proximity with the roadway concealing the primaryconductors. However this option may bemused under other circumstances.

Iron (preferably as laminations) may be used in proximity to either orboth the primary conductors and the power collector in order to improvecoupling, although it is believed that this is not cost-effective overmore turns of cable, and may introduce other problems such as magneticattractive forces and magnetostrictive acoustic signals and vibrations,as well as corrosion problems.

In the present embodiment the only consumers are part of oneorganisation, but if multiple groups of consumers were to use the powersome means to charge for the power taken may be required. In order tocater for that possibility, a microprocessor may be provided at eachcharging station to recognize a particular vehicle as one compatiblewith this system for inductive power transfer, to respond to a requestfor power, and then to energize the primary conductor (perhaps byclosing a contactor) until such time as the vehicle moves off or signalsthat its batteries are charged. Vehicle recognition may be visual (usingnumber-plate image identification) or by, means of radio or infraredsignalling 73. Accounting may involve the use of a conventionaltelephone modem, a stored list of transactions read out directly atintervals, or a cellular telephone Alternatively charging positions maybe widely dispersed in the road environment (such as near trafficlights, etc) and vehicles capable of using the power then carry theirown metering facilities.

This embodiment has the advantages over conventional trolley buses thatit has no need for overhead wiring, no unreliable pantograph or othercurrent collectors to the wiring, and can wait indefinitely or goanywhere within reason between charging stops. At a charging stop it hasthe advantage that functional connection is made without any purposiveact by the driver--e.g. to plug in a connector to a fixed post, wait,then remove it. This is useful from safety and anti-vandal aspects aswell.

The magnetic field emitted by the primary conductors 62, 76 isapproximately 25% of the WHO recommendation for 50 Hz magnetic fieldintensity for human body exposure, and in any case is produced only whena suitable vehicle actually overlies the conductors.

A prototype rubber-tired vehicle according to this embodiment, having apower collector with a pickup coil of dimensions 1.2×2.4 meters; 100turns and no on-board storage was capable of moving one person whileconsuming approximately 30 W of induced power within a "CADAC" brushlessDC motor geared down by 40:1, and was still capable of driving againstan obstacle (with consequent loss of traction) while consuming 800 W oftransferred power. The primary conductor loop comprised nine turns ofcable as described previously, about 100 mm beneath the power collectorcoil, which was driven from an ordinary welding transformer fed from a10A mains outlet with about 40 V AC, @50 Hz.

In a variation the overnight storage area, and/or the entire route orportions of it may also be provided with primary conductors andenergizing means as described above.

VARIATIONS

Although in most applications the power will be directed to an electricmotor, it will be appreciated that the trolley may be provided withlight sources, heating elements, or other "consumers" of electric power.

Although the arrangement in FIG. 2 shows the coil mounted beneath thetray of the trolley, in-board of the rails, it will be apparent that analternative allows the coil to be slightly wider than the rail spacing,with the coil situated on the outside perimeter of the trolley, at aheight just above that of the rails, so that the coil extends across theoutside front and rear of the trolley perimeter, and extends on theoutside of the wheels, to provide an even closer coupling between thecoil and the rails. Such an arrangement would approximate to the coilshown in FIG. 2, which is wider than the rail spacing.

Because of the importance of the skin effect at higher frequencies, therails can be formed from hollow metal sections, or if more solid railsare required, the rails could be formed of composite materials with onlythe outside few millimeters being formed of aluminium or otherconductive material.

Finally, it will be appreciated that various alterations ormodifications may be made to the foregoing without departing from thescope of this invention as set forth in the following claims.

I claim:
 1. An inductive power transfer system comprising:a source ofalternating electric power having an operating frequency; a resonantprimary for generating a magnetic field from said source of alternatingcurrent; a vehicle having at least one inductive power collector mountedthereon; wherein said inductive power collector comprises at least onecapacitance and at least one coil of one or more windings forintercepting the magnetic field from said resonant primary, and at leastone electrical load, wherein the at least one capacitance is in serieswith the coil and the load, and the reactance of the series capacitanceis substantially equal to the sum of (a) the inductive reactance of thecoil at the operating frequency and (b) the mutual reactance of thecoupling between the resonant primary and the at least one coil of thepower collector, and wherein there is no parallel resonance effect. 2.An inductive power transfer system as claimed in claim 1, wherein thecoil of the power collector is spaced apart from the resonant primary byup to about 100 mm.
 3. An inductive power transfer system as claimed inclaim 2, wherein the operating frequency of the source of alternatingpower lies in the range of from 30 Hz to 1 MHz.
 4. An inductive powertransfer system as claimed in claim 3, wherein the operating frequencyof the source of alternating power is that of a local supply frequencyor an integer multiple of the supply frequency.
 5. An inductive powertransfer system as in claim 4, wherein said resonant primary comprisesplural electrically separated, elongated primary conductor loops each ofsaid loops being contiguous at at least one end with an adjacent one ofsaid loops.
 6. An inductive power transfer system as claimed in claim 1,wherein said resonant primary comprises one or more physically separatedloops which are located at designated positions so that the powercollector is energized when said vehicle is in physical proximity to anyone of said loops.
 7. An inductive power transfer system as claimed inclaim 1, wherein the load comprises a device capable of using thealternating current after a first conversion into direct current and asecond conversion into a voltage or current of a regulated amplitude. 8.An inductive power transfer system comprising:a source of alternatingelectric power having an operating frequency; a resonant primary havingan elongated track comprising a pair of electrically conductive rails; aplurality of vehicles each vehicle capable of moving on said track andeach vehicle having at least one inductive power collector mountedthereon; wherein each said inductive power collector comprises at leastone capacitance and at least one coil of one or more windings forintercepting a magnetic field from the primary conductive rails, and atleast one electrical load, wherein the at least one coil is spaced apartfrom the conductive rails by up to about 100 mm, wherein the at leastone capacitance is in series with the coil and the load, and thereactance of the series capacitance is substantially equal to the sum of(a) the inductive reactance of the coil at the operating frequency and(b) the mutual reactance of the coupling between the resonant primaryand the at least one coil of the power collector, and wherein there isno parallel resonance effect thereby allowing said vehicles to beadjacent to one another on said track without interfering with oneanother.
 9. A self-powered vehicle, adapted for use with an inductivepower transfer source producing a varying magnetic field at an operatingfrequency, said vehicle comprising:at least three road wheels and meansfor control and guidance, at least one inductive power collector forcollecting power from said inductive power transfer system, apparatusfor conversion of electrical energy into a storable form, apparatus forstorage of electrical energy, apparatus for recovery of storedelectrical energy and its controlled delivery to at least oneelectromotive transducer, the power collector comprising at least onecapacitance and at least one coil of one or more windings forintercepting the magnetic field from the inductive power transfersource, and at least one electrical load, wherein the at least onecapacitance is in series with the coil and the load, and the reactanceof the series capacitance is substantially equal to the sum of (a) theinductive reactance of the coil at the operating frequency and (b) themutual reactance of the coupling between the inductive power transfersource and the at least one coil of the power collector, and whereinthere is no parallel resonance effect.
 10. A power collector forcollecting power from an inductive power transfer source across a space,using one or more primary conductors capable of carrying an alternatingelectric current having an operating frequency as primary emitters of avarying magnetic field, the power collector comprising at least onecapacitance and at least one coil of one or more windings forintercepting the magnetic field from the one or more primary conductors,and at least one electrical load,wherein the at least one capacitance isin series with the coil and the load, and the reactance of the seriescapacitance is substantially equal to the inductive reactance of thecoil at the operating frequency, and wherein there is no parallelresonance effect.